Conventionally, as a camera focus state detection apparatus, a so-called phase difference focus detection method-type focus state detection apparatus is known. In the phase difference focus detection method, light rays from a subject that pass through different exit pupil areas of an image sensing lens are focused on a pair of line sensors. Then, by obtaining the amount of shift of the relative positions of the pair of subject images obtained by photoelectric conversion of the subject images (hereinafter, this process is called “phase difference computation”), a subject defocus amount is detected and the image sensing lens is driven based on that detected amount of defocus (for example, Japanese Patent Application Laid-Open No. 09-054242).
In addition, a multi-point focus detection apparatus that carries out focus detection of a plurality of subjects is also known. In such an apparatus, the pair of line sensors is divided among a plurality of areas, signal charge accumulation control is carried out for each area, and the pairs of subject images obtained by photoelectric conversion at each of these areas are correlated (for example, Japanese Patent Application Laid-Open No. 2003-215442).
In addition, in the focus state detection apparatus using the phase difference focus detection method, a focus state detection apparatus that can adjust the detectable defocus amount by changing the area of the pair of line sensors used for charge accumulation control and phase difference computation is also known (for example, Japanese Patent Application Laid-Open No. 63-172206; see FIG. 8).
The focus state detection apparatus disclosed in Japanese Patent Application Laid-Open No. 63-172206 makes it possible to select an appropriate charge accumulation control area depending on the focus state detection results and the maximum defocus amount of the image sensing lens. However, where focus state detection does not function, it is necessary to change the charge accumulation control area and to redo the charge accumulation operation and the phase difference computation operation, which lengthens the time required for focus state detection.
In addition, the focus state detection apparatus of Japanese Patent Application Laid-Open No. 63-172206 can also be adapted to a multi-point focus detection apparatus that enables focus state detection of a plurality of subjects like the focus state detection apparatus of Japanese Patent Application Laid-Open No. 2003-215442. As an example of an instance in which such a focus state detection apparatus is created, charge accumulation control areas at three distance measurement points (focus state detection areas) are illustrated in FIGS. 10A and 10B.
FIG. 10A shows the line sensor charge accumulation control areas (L area, C area and R area) when the amount of defocus is small (small defocus amount) and the subject images phase difference (that is, the extent of the displacement between the images) is also small. In the case of a small defocus amount, the phase difference between the subject images is small and the breadth of each area can be narrowed, thus enabling focus state detection of a plurality of subjects from signals obtained from each of these areas.
By contrast, FIG. 10B shows the line sensor charge accumulation control area (W area) when the amount of defocus is large (large defocus amount). The subject images phase difference is large, and therefore it is necessary to use the entire line as a single area in order to prevent the subject image from projecting beyond the boundaries of the area.
Thus, as described above, where the focus state detection apparatus of Japanese Patent Application Laid-Open No. 63-172206 is adapted to the multi-point focus detection apparatus like the focus state detection apparatus of Japanese Patent Application Laid-Open No. 2003-215442, initially a charge accumulation operation is carried out over a divided area like those shown in FIG. 10A even when the amount of defocus is large, and thereafter, if the defocus amount is in fact large, it is then necessary to carry out the charge accumulation operation once again over an area like that shown in FIG. 10B. Consequently, the time needed for focus state detection necessarily lengthens, which is undesirable.